This invention relates to branched polymers containing dendritic branches having functional groups uniformly distributed on the periphery of such branches. This invention also relates to processes for preparing such polymers as well as applications therefore.
Organic polymers are generally classified in a structural sense as either linear or branched. In the case of linear polymers, the repeating units (often called mers) are divalent and are connected one to another in a linear sequence. In the case of branched polymers, at least some of the mers possess a valency greater than 2 such that the mers are connected in a nonlinear sequence.
The term "branching" usually implies that the individual molecular units of the branches are discrete from the polymer backbone, yet have the same chemical constitution as the polymer backbone. Thus, regularly repeating side groups which are inherent in the monomer structure and/or are of different chemical constitution than the polymer backbone are not considered as branches, e.g., dependent methyl groups of linear polypropylene. To produce a branched polymer, it is necessary to employ an initiator, a monomer, or both that possess at least three moieties that function in the polymerization reaction. Such monomer or initiators are often called polyfunctional.
The simplest branched polymers are the chain-branched polymers wherein a linear backbone bears one or more essentially linear pendant groups. This simple form of branching, often called comb branching, may be regular wherein the branches are uniformly and regularly distributed on the polymer backbone or irregular wherein the branches are distributed in nonuniform or random fashion on the polymer backbone. See T. A. Orofino, Polymer, 2, 295-314 (1961). An example of regular comb branching is a comb-branched polystyrene as described by T. Altores et al. in J. Polymer Sci., Part A, 3, 4131-4151 (1965) and an example of irregular comb branching is illustrated by graft copolymers as described by Sorenson et al. in "Preparative Methods of Polymer Chemistry", 2nd Ed., Interscience Publishers, 213-214 (1968).
Another type of branching is exemplified by cross-linked or network polymers wherein the polymer chains are connected via tetravalent compounds, e.g., polystyrene molecules bridged or cross-linked with divinylbenzene. In this type of branching, many of the individual branches are not linear in that each branch may itself contain groups pendant from a linear chain. More importantly in network branching, each polymer macromolecule (backbone) is cross-linked at two or more sites to two other polymer macromolecules. Also the chemical constitution of the cross-linkages may vary from that of the polymer macromolecules. In this so-called cross-linked or network-branched polymer, the various branches or cross-linkages may be structurally similar (called regular cross-linked) or they may be structurally dissimilar (called irregularly cross-linked). An example of regular cross-linked polymers is a ladder-type poly(phenylsilsesquinone) as described by Sorenson et al., supra, at page 390. The foregoing and other types of branched polymers are described by H. G. Elias in Macromolecules, Vol. I, Plenum Press, New York (1977).
There have also been developed polymers having so-called star structured branching wherein the individual branches radiate out from a nucleus and there are at least 3 branches per nucleus. Such star-branched polymers are illustrated by the polyquaternary compositions described in U.S. Pat. Nos. 4,036,808 and 4,102,827. Star-branched polymers prepared from olefins and unsaturated acids are described in U.S. Pat. No. 4,141,847. The star-branched polymers offer several advantages over polymers having other types of branching. For example, it is found that the star-branched polymers may exhibit higher concentrations of functional groups thus making them more active for their intended purpose. In addition, such star-branched polymers are often less sensitive to degradation by shearing which is a very useful property in formulations such as paints, in enhanced oil recovery and other viscosity applications. Additionally, the star-branched polymers have relatively low intrinsic viscosities even at high molecular weight.
Recently, in order to provide polymers which exhibit even greater concentrations of functional groups per unit volume of the polymer macromolecule as well as a more uniform distribution of such functional groups in the exterior regions of the macromolecule than exhibited by conventional star polymers, dendritic polymers were developed. See, for example, U.S. Pat. No. 4,507,466. While such dendritic polymers (often called dendrimers) are more compact than conventional star polymers, they are generally spheroidal in shape. For many applications, such as production of molecular composites, generally rod-like or cylindrically-shaped polymers are desirable.